How to validate Subdomain in JavaScript custom button?

Hello, you can use the following function to check the above subdomain on the custom button :-

function validSubdomain() {
var validCases = /[^a-zA-Z0-9\-]/;
var valueOfSubDomain = 'mySubdomain'
if(validCases.test(valueOfSubDomain)) {
alert("invalid Subdomain");
}
else{
alert("valid Subdomain");
}
}

Hope this will work.